Log Classes Overview

Introduction

This is a general overview of logging classes provided by wxPython. The word logging here has a broad sense, including all of the program output, not only non-interactive messages. The logging facilities included in wxPython provide the base wx.Log class which defines the standard interface for a log target as well as several standard implementations of it and a family of functions to use with them.

First of all, no knowledge of wx.Log classes is needed to use them. For this, you should only know about wx.LogDebug, wx.LogError, wx.LogMessage and similar functions. All of them have the same syntax as the Python logging module.

Here are all of them:

• wx.LogFatalError which is like wx.LogError, but also terminates the program with the exit code 3 (using abort() standard function). Unlike for all the other logging functions, this function can’t be overridden by a log target.
• wx.LogError is the function to use for error messages, i.e. the messages that must be shown to the user. The default processing is to pop up a message box to inform the user about it.
• wx.LogWarning for warnings. They are also normally shown to the user, but don’t interrupt the program work.
• wx.LogMessage is for all normal, informational messages. They also appear in a message box by default (but it can be changed, see below).
• wx.LogVerbose is for verbose output. Normally, it is suppressed, but might be activated if the user wishes to know more details about the program progress (another, but possibly confusing name for the same function is wx.LogInfo).
• wx.LogStatus is for status messages. They will go into the status bar of the active or specified (as the first argument) wx.Frame if it has one.
• wx.LogSysError is mostly used by wxPython itself, but might be handy for logging errors after system call (API function) failure. It logs the specified message text as well as the last system error code (errno or Windows’ GetLastError() depending on the platform) and the corresponding error message. The second form of this function takes the error code explicitly as the first argument.
• wx.LogDebug is the right function for debug output. It only does anything at all in the debug mode (when the preprocessor symbol __WXDEBUG__ is defined) and expands to nothing in release mode (otherwise). Note that under Windows, you must either run the program under debugger or use a 3rd party program such as DebugView (http://www.microsoft.com/technet/sysinternals/Miscellaneous/DebugView.mspx) to actually see the debug output.
• wx.LogTrace as wx.LogDebug only does something in debug build. The reason for making it a separate function from it is that usually there are a lot of trace messages, so it might make sense to separate them from other debug messages which would be flooded in them. Moreover, the second version of this function takes a trace mask as the first argument which allows to further restrict the amount of messages generated.

The usage of these functions should be fairly straightforward, however it may be asked why not use the other logging facilities, such as the Python logging module. The short answer is that they’re all very good generic mechanisms, but are not really adapted for wxPython, while the log classes are. However every project is different with different needs, so you are encouraged to investigate all options and use what works best for you

Some of advantages in using wxPython log functions are:

• Portability
• Flexibility: The output of wx.Log functions can be redirected or suppressed entirely based on their importance, which is either impossible or difficult to do with traditional methods. For example, only error messages, or only error messages and warnings might be logged, filtering out all informational messages.
• Completeness: Usually, an error message should be presented to the user when some operation fails. Let’s take a quite simple but common case of a file error: suppose that you’re writing your data file on disk and there is not enough space. The actual error might have been detected inside wxPython code, so the calling function doesn’t really know the exact reason of the failure, it only knows that the data file couldn’t be written to the disk. However, as wxPython uses wx.LogError in this situation, the exact error code (and the corresponding error message) will be given to the user together with “high level” message about data file writing error.

Log Messages Selection

By default, most log messages are enabled. In particular, this means that errors logged by wxPython code itself (e.g. when it fails to perform some operation) will be processed and shown to the user. To disable the logging entirely you can use wx.Log.EnableLogging method or, more usually, wx.LogNull class which temporarily disables logging and restores it back to the original setting when it is destroyed.

To limit logging to important messages only, you may use wx.Log.SetLogLevel with e.g. wx.LOG_Warning value – this will completely disable all logging messages with the severity less than warnings, so wx.LogMessage output won’t be shown to the user any more.

Moreover, the log level can be set separately for different log components. Before showing how this can be useful, let us explain what log components are: they are simply arbitrary strings identifying the component, or module, which generated the message. They are hierarchical in the sense that “foo/bar/baz” component is supposed to be a child of “foo”. And all components are children of the unnamed root component.

By default, all messages logged by wxPython originate from “wx” component or one of its subcomponents such as “wx/net/ftp”, while the messages logged by your own code are assigned empty log component. To change this, you need to define wx.LOG_COMPONENT to a string uniquely identifying each component, e.g. you could give it the value “MyProgram” by default and re-define it as “MyProgram/DB” in the module working with the database and “MyProgram/DB/Trans” in its part managing the transactions. Then you could use wx.Log.SetComponentLevel in the following ways:

# Disable all database error messages, everybody knows databases never
# fail anyhow
wx.Log.SetComponentLevel("MyProgram/DB", wx.LOG_FatalError)

# but enable tracing for the transactions as somehow our changes don't
# get committed sometimes
wx.Log.SetComponentLevel("MyProgram/DB/Trans", wx.LOG_Trace)

# also enable tracing messages from wxPython dynamic module loading
# mechanism
wx.Log.SetComponentLevel("wx/base/module", wx.LOG_Trace)

Notice that the log level set explicitly for the transactions code overrides the log level of the parent component but that all other database code subcomponents inherit its setting by default and so won’t generate any log messages at all.

Log Targets

After having enumerated all the functions which are normally used to log the messages, and why would you want to use them, we now describe how all this works.

wxPython has the notion of a log target: it is just a class deriving from wx.Log. As such, it implements the virtual functions of the base class which are called when a message is logged. Only one log target is active at any moment, this is the one used by LogXXX functions. The normal usage of a log object (i.e. object of a class derived from wx.Log) is to install it as the active target with a call to SetActiveTarget() and it will be used automatically by all subsequent calls to LogXXX functions.

To create a new log target class you only need to derive it from wx.Log and override one or several of wx.Log.DoLogRecord, wx.Log.DoLogTextAtLevel and wx.Log.DoLogText in it. The first one is the most flexible and allows you to change the formatting of the messages, dynamically filter and redirect them and so on – all log messages, except for those generated by wx.LogFatalError, pass by this function. wx.Log.DoLogTextAtLevel should be overridden if you simply want to redirect the log messages somewhere else, without changing their formatting. Finally, it is enough to override wx.Log.DoLogText if you only want to redirect the log messages and the destination doesn’t depend on the message log level.

There are some predefined classes deriving from wx.Log and which might be helpful to see how you can create a new log target class and, of course, may also be used without any change. There are:

• wx.LogStderr: This class logs messages to the C stderr stream.
• wx.LogGui: This is the standard log target for wxPython applications (it is used by default if you don’t do anything) and provides the most reasonable handling of all types of messages for given platform.
• wx.LogWindow: This log target provides a “log console” which collects all messages generated by the application and also passes them to the previous active log target. The log window frame has a menu allowing user to clear the log, close it completely or save all messages to file.
• wx.LogBuffer: This target collects all the logged messages in an internal buffer allowing to show them later to the user all at once.
• wx.LogNull: The last log class is quite particular: it doesn’t do anything. The objects of this class may be instantiated to (temporarily) suppress output of LogXXX functions.

The log targets can also be combined: for example you may wish to redirect the messages somewhere else (for example, to a log file) but also process them as normally. For this the wx.LogChain, wx.LogInterposer, and wx.LogInterposerTemp can be used.

Logging in Multi-Threaded Applications

Starting with wxPython 2.9.1, logging functions can be safely called from any thread. Messages logged from threads other than the main one will be buffered until wx.Log.Flush is called in the main thread (which usually happens during idle time, i.e. after processing all pending events) and will be really output only then. Notice that the default GUI logger already only output the messages when it is flushed, so by default messages from the other threads will be shown more or less at the same moment as usual. However if you define a custom log target, messages may be logged out of order, e.g. messages from the main thread with later timestamp may appear before messages with earlier timestamp logged from other threads. wx.Log does however guarantee that messages logged by each thread will appear in order in which they were logged.

Also notice that wx.Log.EnableLogging and wx.LogNull class which uses it only affect the current thread, i.e. logging messages may still be generated by the other threads after a call to EnableLogging(False).

Logging Customization

To completely change the logging behaviour you may define a custom log target. For example, you could define a class inheriting from wx.Log which shows all the log messages in some part of your main application window reserved for the message output without interrupting the user work flow with modal message boxes.

To use your custom log target you may either call wx.Log.SetActiveTarget with your custom log object or create a wx.AppTraits -derived class and override wx.AppTraits.CreateLogTarget virtual method in it and also override wx.App.CreateTraits to return an instance of your custom traits object. Notice that in the latter case you should be prepared for logging messages early during the program startup and also during program shutdown so you shouldn’t rely on existence of the main application window, for example. You can however safely assume that GUI is (already/still) available when your log target as used as wxPython automatically switches to using wx.LogStderr if it isn’t.

There are several methods which may be overridden in the derived class to customize log messages handling: wx.Log.DoLogRecord, wx.Log.DoLogTextAtLevel and wx.Log.DoLogText.

The last method is the simplest one: you should override it if you simply want to redirect the log output elsewhere, without taking into account the level of the message. If you do want to handle messages of different levels differently, then you should override wx.Log.DoLogTextAtLevel.

Additionally, you can customize the way full log messages are constructed from the components (such as time stamp, source file information, logging thread ID and so on). This task is performed by wx.LogFormatter class so you need to derive a custom class from it and override its Format() method to build the log messages in desired way. Notice that if you just need to modify (or suppress) the time stamp display, overriding FormatTime() is enough.

Finally, if even more control over the output format is needed, then LogRecord() can be overridden as it allows to construct custom messages depending on the log level or even do completely different things depending on the message severity (for example, throw away all messages except warnings and errors, show warnings on the screen and forward the error messages to the user’s (or programmer’s) cell phone – maybe depending on whether the timestamp tells us if it is day or night in the current time zone).

Using trace masks

Notice that the use of log trace masks is hardly necessary any longer in current wxPython version as the same effect can be achieved by using different log components for different log statements of any level. Please see Log Messages Selection for more information about the log components.

The functions below allow some limited customization of wx.Log behaviour without writing a new log target class (which, aside from being a matter of several minutes, allows you to do anything you want). The verbose messages are the trace messages which are not disabled in the release mode and are generated by wx.LogVerbose. They are not normally shown to the user because they present little interest, but may be activated, for example, in order to help the user find some program problem.

As for the (real) trace messages, their handling depends on the currently enabled trace masks: if wx.Log.AddTraceMask was called for the mask of the given message, it will be logged, otherwise nothing happens.

For example:

wx.LogTrace(wx.TRACE_OleCalls, "Foo.Bar() called")

will log the message if it was preceded by:

wx.Log.AddTraceMask(wx.TRACE_OleCalls)

The standard trace masks are given in the wx.LogTrace documentation.